Direct bone conductors are essential for the rehabilitation of patients suffering from some specific type of hearing losses for which traditional hearing aids are insufficient.
This type of device usually consists of an external hearing aid with a vibrator that is connected via a coupling to a skin-penetrating abutment mounted on a fixture anchored in the skull bone. The coupling allows the hearing aid to be easily connected and disconnected from the abutment.
Typical for all direct bone conductors is that the vibrator of the device is directly connected to a fixture that is anchored in the skull bone so that the damping of the vibrations from the vibrator to the skull bone is negligible.
Direct bone conductors are mainly been used to rehabilitate patients with conductive or unilateral hearing losses who have a quite mild sensorineural hearing loss component.
When measuring a direct bone conductor the output from the hearing aid is vibrations that are measured in dB OFL rel 1 μN (decibel Output Force level relative 1 micro Newton). The input to the hearing aid is sound, which is measured in dB SPL (decibel sound pressure level relative 20 μPa). No feedback shall be present at the measurement setting. A standard equipment for measuring direct bone conductors is the Skull simulator TU1000, P&B research AB, Sweden.
We here define a hearing aid as a direct bone conductor that fulfills both the following two criteria:    1. A direct bone conductor that, at an input frequency sweep of 60 dB SPL, is able to perform an output for which the average of the output values for 1600 Hz and 2000 Hz is greater than 98 dB OFL (rel 1 μN)    2. A direct bone conductor that is able to perform a maximum output for which the average of the maximum output values for 1600 Hz and 2000 Hz is greater than 109 dB OFL (rel 1 μN).
Direct bone conductors for more severe sensorineural hearing loss components are available. However, these powerful direct bone conductors have several drawbacks. Due to the powerful output the patients often experience acoustic feedback problems with this kind of device. These patients also have a more severe sensorineural hearing loss component so they have a more limited dynamic range and often also a more frequency dependent hearing loss compared to patients who mainly have a conductive hearing loss. Existing powerful direct bone conduction hearing aids are based on analog amplifiers and the patient's ability to hear well is limited since the hearing aid cannot be sufficiently well adapted to compensate for the patients individual hearing loss and different sound environments. Existing powerful direct bone conduction hearing aids only use traditional omni directional microphones. This means that the possibilities for the patients to understand speech in noisy environments are limited.